Preparation of photographic silver halide emulsions



United, atcsPat PREPARATION OF PHOTOGRAPHIC SILVER HALIDE EMULSIONS Alfons Jozef De Pauw, Wilrijk-Antwerp, and Ren Camille Gerbaux, Edegem-Antwerp, Belgium, assignors to Gevaert Photo-Production, N .V., Mortsel-Autwerp, Belgium, 2 Belgian company No Drawing. Filed June 6, 1957, Ser. No. 663,885

Claims priority, application Great Britain June 7, 1956 4 Claims. (Cl. 96-413) The present invention relates to the preparation of photographic silver halide emulsions and of photographic material, such as paper, plates and films, incorporating a layer of such an emulsion.

In the preparation of photographic emulsions, it is commonly known to use gelatin as a protective colloid for the silver halides, on account of its great peptizing power, good water-permeability and exceptional thermo-reversible gel-formation property. Thus, the silver halide is formed inyan aqueous gelatin solution and an additional amount of gelatin may be added after formation of the dispersion, in order to solidify, noodle and Wash the emulsion.

l However, gelatin has many disadvantages which place restrictions upon its use. Thus, gelatin is susceptible to attack by animal and vegetable organisms. Insects attack it, especially in the tropics, and it is also susceptible to' the action of moulds and bacteria, particularly under conditions of high humidity and when the climate is favourable for such organisms. that, owing to the natural orgin of gelatin, a supply from agiven source often varies in properties from time to time. Furthermore emulsion coats of gelatin become very soft' in Warm water, hence processing solutions must 2,956,883 Patented Oct. 18, 19.69

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A further object of the present invention is to prepare silver halideemulsions with high light-sensitivity even when using polyvinyl alcohol as binding agent.

Still another object of the present invention is the ripening of silver halide emulsions in synthetic water-permeable colloid solutions.

Further objects of the present invention will appear from the following description.

We have found that cellulose sulphates as further defined hereinafter are excellent protective colloids in which silver halide suspensions with excellent photographic properties can easily be prepared.

The term cellulose sulphate as used hereinbefore is meant to cover cellulose esters containing only sulphate groups and also mixed cellulose ether sulphates especially such as for instance hydroXy-ethyl cellulose sulphate, methyl cellulose sulphate, beta-sulpho-ethyl cellulose, and carboxy-methyl cellulose sulphate.

The pure cellulose sulphates considered for the present invention are water-soluble salts such as sodium-, potassiumand ammonium-cellulose sulphates. They may be obtained according to US. Nos. 2,559,914 and 2,539,451 by reacting sulphuric acid upon cellulose in the presence of an aliphatic alcohol. The reaction may be carried out at temperatures between 0 and 25 C. By neutralizing with an alkali, the corresponding desired salt is obtained. The degree of esterification may be regulated by varying the concentration of the reagents and the time of reaction, and the viscosity of the endproducts in water depends on the degree of polymerization of the cellulosic starting materials, on the degrada Another difliculty is be'kept cold; also, when exposed to a dry air, such coats advantagesof such synthetic binders include: failure to I allow a growth of the silver halide crystals, liability to cause flocculation of the emulsion and insuflicient water- 'permeability. i The use of polyvinyl alcohol has been tried on account of its good layer-forming properties, and layers formed from aqueous polyvinyl alcohol solutions are transparent, show good physical properties and are sufficiently waterpermeable. However, it is impossible to prepare coarsegrained silver halide emulsions directly in solutions of polyvinyl alcohol. Polyvinyl alcohol prevents the growth of the silver halide crystals, so that emulsions are obtained with an extremely fine grain and consequently with very low light-sensitivity.

Several other synthetic polymers have been tested also as peptizing agents for silver halides such as polyvinyl sufiicient protective properties so that emulsions prepared therein flocculate.

It is an object of the present invention to suspend silver halide in a solution of a water-permeable colloid.

Another object of the present invention is to produce in a simple way silver'halide free from water-soluble salts. 4

tion brought by the conditions of the reaction and on the degree of estenification of the sulphate. Cellulose sulphate with an average degree of polymerization and a substitution of about 0.5 sulphate groups per glucose unit gives quite satisfactory results.

, As cellulose ether sulphates can be used all water-soluble derivatives such as for instance methyl cellulose sulphate, ethyl cellulose sulphate, ethoxy-ethyl cellulose sulphate, hydroxy-ethyl cellulose sulphate, ethyl-hydroxyethyl cellulose sulphate etc. with varying substitution degrees of ether functions and sulphate groups. In the case of hydroxy-eth-yl cellulose sulphate, the sulphate groups can esterify the hydroxyl groups of the cellulose chain and/or the primary hydroxyl groups of the ether function.

These derivatives can be obtained in the same w-ayas cellulose sulphate by the action of sulphuric acid upon the cellulose ether in the presence of an aliphatic alcohol. The reaction circumstances: relative amounts of the reagents, temperature, duration etc. must be chosen in function of the nature of the starting products and of the desired structure and properties of the end productl An example of a cellulose sulpho-ether is beta-sulphoethyl cellulose which under the form of an alkali metal salt and with a substitution degree of 0.2 is water-soluble. This product can be prepared according to the methods known in the art, i.e. the action of sodium-beta-bromm ethyl sulphonate upon alkali cellulose.

Unlike the above polymeric substances the cellulose sulphates according to the present invention allow as protective colloids the growth of silver halide crystals and give non-flocculated suspensions with high light-sensitivity.

According to the present invention, aqueous solutions of at least one silver salt, for instance silver nitrate, and of at least one halide salt such as sodium chloride or potassium bromide, are mixed in the presence of the cellulose sulphates. This may be done by simultaneously pouring, whilst stirring, the salt solutions in an aqueous solution of the cellulose sulphates. In this way a suit:

able suspension of silver halide is obtained. To this suspension may be added further amounts of the cellulose sulphates or suitable high-molecular compounds which are miscible with the cellulose sulphates and possess the desired properties.

It is a particular advantage of the present invention that the cellulose sulphates are completely miscible with polyvinyl alcohol. The latter possesses particularly good layer-forming properties. Thus after physical ripening, the sensitive coarse-grained suspensions of silver halide in the cellulose sulphates may be converted by the addition of polyvinyl alcohol into useful photographic emulsions, the coated layers of which possess high physical properties.

When gelatin is used as binding agent, the emulsions can be gelled, noodled and washed in the usual way. If, when using polyvinyl alcohol, the emulsion is to be washed, gelling agents such as alpha-naphthol or gallic acid can be added, whereby on cooling a gel is obtained which may be washed after noodling.

The soluble salts may be separated from the suspension by dialysis, i.e. by diffusion through a semi-permeable membrane out of the emulsion to a solution with a low salt content.

A further method for obtaining method for obtaining emulsions free from water-soluble salts comprises precipitating a suspension of silver halide in the cellulose sulphates according to the present invention in a coagulating medium such as a water-miscible organic solvent, e.g. acetone, methanol, ethanol, methyl acetate and ethyl acetate, or in a concentrated aqueous solution of at least one inorganic salt such as sodium sulphate or sodium chloride. The precipitate thus obtained in finely divided and may easily be freed from the Water-soluble salts formed as by-products in the preparation of the silver halide by repeated washing with mixtures of water and organic solvents and decanting.

The powdery precipitate thus Washed forms, on addition of water, a stable suspension to which the necessary amount of binding agent, e.g. polyvinyl alcohol, may be added. Other synthetic, half synthetic, or natural polymers which are miscible with the cellulose sulphates and possess bood layer-forming properties may also be used, e.g. gelatin.

Any desired amount of water and binding agent may be added to the washed powdery precipitate of silver halide emulsions with Widely varying concentration, e.g. an emulsion with a high silver halide content.

In the preparation of silver halide suspensions, a starting solution of the cellulose sulphates with a concentration of about 0.3% is suflicient, but greater amounts may be used. After establishing the desired grain distribution of the silver halide by physical ripening in a solution of the cellulose sulphates and, if necessary, freeing the emulsion from excess of soluble salts, the light-sensitivity of the silver halide crystals may be increased by chemical sensitization. Some chemical sensitizers may also be added at the precipitation stage of the silver halide or during or after the ripening period, e.g. when before washing, a naturally active gelatin is added as binding agent for the silver halide.

It is well-known to sensitize emulsions chemically with so-called sulphur sensitizers, which possibly form silver sulphide nuclei with compounds appearing to form silver nuclei, or with compounds of gold or other heavy metals such as palladium and platinum.

Thus, in order to sensitize chemically with sulphur sensitizers, the emulsion may be digested with naturally active gelatin, or sulphur compounds such as allyl isothiocyanate, allylthiourea, sodium thiosulphate or potassium selenocyanide may be added.

The emulsions may also be chemically sensitized with compounds apparently forming silver nuclei, e.g. stannous salts, or imino-amino-methane sulphinic acid compounds 4 such as those described in the co-pending U.S.A. patent application Serial No. 581,315.

The emulsions may also be treated in the known manner With compounds of gold or other heavy metals such as ruthenium, rhodium, palladium, iridium and platinum, all of which belong to group VIII of the periodic table of elements and have an atomic Weight greater than 100. Surprisingly it has been discovered that the process of emulsion preparation according to the present invention has many advantages over the known methods regarding the sensitization with such compounds.

Thus, although the speed-increasing properties of gold compounds have been known for a long time (Fr. Kropfi, Photographische Industrie 23 (1925), 1145) and such compounds have been widely used for enhancing the sensitivity of many gelatino-silver halide emulsions, the chemical sensitization of gelatino-silver halide emulsions by the compounds of the heavy metals such as platinum and palladium belonging to group VIII of the periodic table has hitherto not found general application.

Probably this fact may be brought into relation with the difference between the gold compounds and the compounds of other heavy metals such as palladium and platinum with respect to their sensitizing properties in the presence of gelatin. As described by P. Faelens, Science et Industries Photographiques 27 (1956), 4/7 and 121/122, this difference should result from the thermoreversibility or -irreversibility of the inhibiting action exerted by the gelatin. Thus, although the inhibited sensitizing power of the gold compounds may partially be regenerated by digestion at 4050 C., the reduced sensitizing power of other heavy metals such as platinum and palladium, should be practically unaffected by such a thermal process. On the contrary, in the absence of gelatin or other protein binder for the silver halide, great speed increases may easily be obtained by adding to the emulsion even small quantities of compounds of heavy metals such as platinum and palladium. With respect to the addition of gold compounds in the same conditions, digestion at raised temperature is no longer necessary for obtaining high-sensitivity emulsions and the quantities to be added are much smaller than in the presence of gelatin.

Thus, the process of emulsion preparation according to the present invention is particularly advantageous in that it provides a practical and economical method of emulsion preparation without using gelatin or other protein for the peptization of the silver halide, so that compounds of gold or heavy metals belonging to group VIII of the periodic table may advantageously be used as speed-increasing agents.

In practising the chemical sensitization method as hereinbefore described, the compounds of heavy metals may advantageously be added, after removing the excess of water-soluble salts, to the re-suspended silver halide/ cellulose sulphate precipitate, to the re-melted silver halide/ polyvinyl alcohol noodles or to the emulsion which is obtained after polyvinyl alcohol (or other layer-forming polymer) is added to said re-suspended precipitate or re-melted noodles. It is to be understood that gelatin may also be used as layer-forming polymer, but in this case the gelatin is preferably added after the addition of the compounds of heavy metals.

In the preparation of emulsions in accordance with the present invention, ingredients such as optical sensitizing dyes, stabilizers, anti-fogging agents, color couplers, hardening agents, and wetting agents, may be added to the emulsion in any convenient manner.

The following examples illustrate the present invention without limiting the scope thereof. All parts are given by weight.

Example I To a solution of 2 parts of sodium cellulose sulphate in 280 parts of distilled water and 50 parts of concentrated ammonia, the following two solutions were slowly and simultaneously added at 40 C. whilst stirringfand in the absence of actinic light: A. 50 parts of silver nitrate dissolved in 85 parts of distilled water, 7 B. 43 parts potassium Bromide and 1 part potassium iodide dissolved in 100 parts of distilled water.

The emulsion was ripened for about half an hour at the same. temperature. After ripening, the emulsion was precipitated with acetone, the supernatant liquid decanted and the precipitate repeatedly washed with a mixture of acetone and .water. in' about equal proportions and decanted. After washing, the precipitate was added to 450 parts distilled water and, afterdissolv-ing 500 parts of a 10% polyvinyl alcohol solution in water were mixed therewith. .;A ft er';thorough stirring, the emulsion was chemically ripened, with sodium thiosulphate. The finished emulsion was coated onto cellulose triacetate film. The photographic material thus obtained was relatively high-sensitive and could be normally exposed, developed, fixed and rinsed similarly to the photographic papers and films wherein gelatin is used as protective colloid.

' Example 2 100 parts of silver nitrate, dissolved in a mixture of 80 parts of concentrated ammonia and 85 parts of distilled water, were slowly added to a solution of 50 parts of potassium bromide, 20 parts of ammonium bromide and 5 parts of potassium iodide in 450 parts of a 1% aqueous solution of sodium cellulose sulphate diluted with 200 parts of distilled water, whilst stirring, and in the absence of actinic light at 25 C.

The emulsion was ripened for 20 minutes at 30 C. Once the desired grain size was obtained, 1000 parts of a aqueous solution of polyvinyl alcohol were added. Whilst thoroughly stirring, 3.5 parts of alpha-naphthol dissolved in 25 parts of ethanol were added. The emulsion jellified on cooling, and after noodling, was washed for 2 hours in running water. After washing, the emulsion was re-melted and chemically ripened in the presence of sodium thiosulphate. The finished emlusion was coated onto baryta paper. The treatment in the processing baths may be the same as for the usual photographic materials with gelatin emulsions.

Example 3 A solution of 25 parts of silver nitrate in parts of concentrated ammonia and 20 parts of distilled water and a solution of 22 parts of potassium bromide in 25 parts of distilled water, were slowly and simultaneously added whilst stirring to a solution of 1 part of sodium cellulose sulphate and 1.25 parts potassium iodide in 115 parts of distilled Water, within 7 minutes at 50 C. The total ripening time was 40 minutes. The silver bromide grains had an average diameter of 0.5;. After ripening, 200 parts of an aqueous polyvinyl alcohol solution 10% and 50 parts of distilled water were added.

After addition of 2 parts of alpha-naphthol, dissolved in 10 parts of ethanol, the emulsion jellified by cooling. The noodled emulsion was washed for two hours and a half in running water. Chemical ripening was done with sodium thiosulphate. The finished emulsion was coated onto cellulose triacetate film and could be treated in the normal processing baths.

Example 4 To a solution of 2 parts of sodium cellulose sulphate in 200 parts of water and 20 parts of concentrated ammonia, the two following solutions were simultaneously added at 50 C. whilst stirring within 3 minutes and in the absence of actinic light:

A. 50 parts of silver nitrate dissolved in 80 parts of concentrated ammonia,

B. 35 parts of ammonium bromide and 1 part of potassium iodide in 110 parts of distilled water.

The emulsion was ripened for 30 minutes at 50 C. and poured into an excess of an aqueous sodium sulphate solution 10%. After decanting the supernatant liquid, the precipitate was repeatedly washed with a mixture of water and alcohol. Next, 300 parts of distilled water were added and the whole heated at 35-40 C. until completely dissolved. 500 parts of a 10% aqueous polyvinyl alcohol solution were then added. The emulsion'was chemically-ripened in the presence of a gold chloride solution and coated onto a suitable support. This photographic material may be treated in the same wayas ordinary gelatin emulsions.

Example 5 50 parts of silver nitrate dissolved in parts of con- .,,centrated ammonia,

B. 35 parts'of ammonium bromide and 1 part of potassiu'm' iodide in parts of distilled water.

The-emulsion was ripened for 30 minutes at 50 C. and poured into an excess of an aqueous sodium sulphate solution 10%. After decanting the supernatant liquid, the precipitate was repeatedly washed with a mixture of water and alcohol. Next, 300 parts of distilled water were added and the whole heated at 35-40" C. until completely dissolved. 60 parts of gelatin dissolved in 450 parts of distilled water were then added. The emulsion was chemically ripened in the presence of the natural active substances present in the gelatin and coated onto a suitable support. This photographic material may be treated in the same way as ordinary gelatin emulsions.

Example 6 To a solution of 2 parts of hydroxy-ethyl cellulose sulphate (3.76 ethoxy groups and 0.34 sulphate groups per glucose unit) in 260 parts of distilled water the two following solutions A and B, previously brought at 50 C., were simultaneously added whilst stirring within three minutes and in the absence of actinic light, giving a priority of some seconds for the addition of solution A.

A. 43 parts of potassium bromide and 2 /2 parts of potassium iodide in 106 parts of distilled water.

B. 50 parts of silver nitrate dissolved in 41 parts of concentrated ammonia and 43 parts of distilled water.

The suspension thus obtained was ripened at 50 C. for 15 minutes, and immediately thereafter poured out into an excess of acetone. A grainy precipitate was obtained which could easily be washed out with a mixture of equal amounts water and acetone. After washing, the precipitate was brought into 400 parts of Water and dispersed again by thorough stirring. A stable suspension was obtained. To this suspension were added 450 parts of a 10% polyvinyl alcohol solution in water and the mixture was homogenized. The emulsion was chemically ripened at 50 C. by means of the reducing substances present in the polyvinyl alcohol. In this way a sensitve and rather hard emulsion was obtained which was: coated onto paper. this way could be processed in the same way as the ordinary gelatin silver halide papers.

Example 7 If in the process according to Example 6, hydroxy* ethyl cellulose sulphate were replaced by methyl cellulosesulphate (1.85 methyl groups and 0.13 sulphate groups per glucose units), equivalent photographic material; could be prepared with the emulsion obtained.

We claim:

1. A process for the production of silver halide photographic emulsions, which comprises precipitating and dis- The photographic paper prepared in.

persing silver halide in an aqueous solution of a cellulose compound selected from the group consisting of cellulose sulphate, alkyl-cellulose sulphate and hydroxy-alkylcellulose sulphate, said cellulose compound having at least about 0.13 sulphate groups per glucose unit, ripening the silver halide dispersion in said solution, and adding a water-permeable colloid selected from the group consisting of gelatin and polyvinyl alcohol.

2. A process for the production of silver halide photographic emulsions, which comprises precipitating and dispersing silver halide in a dispersing medium consisting of a solution of an aqueous cellulose compound selected from the group consisting of cellulose sulphate, alkylcellulose sulphate, and hydroxy-alkyl-cellulose sulphate, said cellulose compound having at least about 0.13 sulphate groups per glucose unit, ripening the silver halide dispersion in said solution, coagulating and washing the silver halide, and redispersing the said precipitate in an aqueous medium containing a colloid selected from the group consisting of gelatin and polyvinyl alcohol.

3. A process for the production of silver halide photographic emulsions, which comprises precipitating silver halide in a dispersing medium consisting of a solution of an aqueous cellulose compound selected from the group consisting of cellulose sulphate, alkyl-cellulose sulphate,

9 9 h rq -a kyll ulo e ph te, said ellul se. com pound having at least about 0.13 sulphate groups per glucose unit, ripening the silverhalide dispersion and adding directly before the ripening to said dispersion a water-permeable colloid selected from the group consisting of gelatin and polyvinyl alcohol.

4. A photographic silver halide emulsion comprising silver halide particles dispersed in a mixture of a cellulose compound selected from the group consisting of cellulose sulphate, alkyl-cellulose sulphate, and hydroxy-alk'yl-cel! lulose sulphate, and a water-permeable colloid selected from the group consisting of gelatin and polyvinyl alcohol, said cellulose compound having at least about 0.13 sulphate group per glucose unit.

References Cited in the file of this patent UNITED STATES PATENTS 11,266 Cutting July 11, 1854 2,127,573 Sheppard et al Aug. 23, 1938 2,322.085 Yutzy June 15, 1943 FOREIGN PATENTS 496,049 Great Britain Nov. 21, 1938 

1. A PROCESS FOR THE PRODUCTION OF SILVER HALIDE PHOTOGRAPHIC EMULSIONS, WHICH COMPRISES PRECIPITATING AND DISPERSING SILVER HALIDE IN AN AQUEOUS SOLUTION OF A CELLULOSE COMPOUND SELECTED FROM THE GROUP CONSISTING OF CELLULOSE SULPHATE, ALKYL-CELLUSLOSE SULPHATE AND HYDROXY-ALKYLCELLULOSE SULPHATE, SAID CELLULOSE COMPOUND HAVING AT LEAST ABOUT 0.13 SULPHATE GROUPS PER GLUCOSE UNIT, RIPENING THE SILVER HALIDE DISPERSION IN SAID SOLUTION, AND ADDINIG A WATER-PERMEABLE COLLOID SELECTED FROM THE GROUP CONSISTING OF GELATIN AND POLYVINYL ALCOHOL. 